This invention pertains generally to tunable bandpass filters for radio frequency energy, and particularly to a tunable microwave bandpass filter having a passband of frequencies wherein a frequency-locked loop is used to track an input signal and to control the filter to keep the center frequency of the passband coincident with the frequency of the input signal.
Filters employing YIG spheres are used extensively as electronically tunable microwave bandpass filters because such filters offer narrow bandwidth, low insertion loss and a wide frequency tuning range. In typical applications of such filters, the following characteristics of the passband of the filter used are necessary: (a) accurate positioning relative to the frequency of an input signal; (b) stability over a period of time and during changes of environmental conditions; and (c) a highest possible frequency slewing rate.
One of the major problems associated with YIG filters is the drift of the center frequency of the passband due to changing temperature. Such drift is caused primarily by the magnetic circuit associated with the YIG filter; more specifically, by changes of dimensions of the mechanical configuration of the electromagnet used in any known YIG filter. A known technique for reducing drift in YIG filters is to arrange for the resonant frequency of the YIG sphere to be dependent on temperature by changing the relative position of the YIG sphere in the magnetic field. By rotating the YIG sphere, the effect of changing temperature can be minimized. Although this reduces the amount of drift, the drift of the center frequency of the passband may still be appreciable.
If a YIG filter only requires a limited tuning range, then a permanent magnet can be used together with the electromagnet. In such a filter, stabilization is improved by placing a ferrite sleeve around the permanent magnet to provide a temperature-dependent shunt to the magnetic field. While such technique reduces the drift of the center frequency of the passband, a satisfactory degree of stabilization is difficult to achieve without following an extensive procedure for alignment of the elements in the YIG filter. Consequently, the cost of YIG filters may be increased substantially.
The above-mentioned techniques are effective only after all parts of the YIG filter have reached a uniform temperature. Changes in ambient temperature cause temperature gradients to be engendered inside the YIG filter, such gradients depending upon the rate at which the ambient temperature is changing. A rapid change in ambient temperature causes any of the above techniques to be ineffective.
A second major problem associated with YIG filters is the drift of the center frequency of the passband due to the effects of microphonism. YIG filters are often used in environments wherein mechanical vibrations are experienced. In such environments the center frequency of the passband of any known YIG filter will vary because mechanical vibrations result in different displacements of the YIG sphere and the elements that produce the magnetic field within the YIG filter. As a result, then, the relationship between the YIG sphere and the magnetic field within the YIG filter changes, the ultimate effect then being an unwanted change in the center frequency of the passband of the YIG filter.